1. Field of the Invention
The present invention relates to a method used in a communication device in wireless communication system, and more particularly, to a method of handling frequency de-prioritization.
2. Description of the Prior Art
Based on 3GPP specification of Radio Resource Control (RRC) protocol, when a user equipment (UE) receives the RRCConnectionReject message including a parameter, deprioritisationTimer, the UE shall start or restart timer t325 with the timer value set to the deprioritisationTimer signaled. deprioritisationTimer indicates the period for which the signalled carrier or E-UTRA is deprioritized, with a value in minutes.
In addition, the UE shall store another parameter, deprioritisationReq, included in RRCConnectionReject message, until t325 expiry, irrespective of any other cell reselection absolute priority assignments (by dedicated or common signalling), or RRC connections in Evolved Universal Terrestrial Radio Access Network (E-UTRAN) or other radio access technologies (RATs). The deprioritisationReq contains “frequency” indicating a currently used carrier frequency is deprioritized in cell reselection or contains RAT, e.g. “e-utra” indicating all frequencies of E-UTRA are deprioritized in cell reselection. If another deprioritisationReq is received before t325 expiry, the UE shall be able to store 8 deprioritisationReq.
Moreover, based on 3GPP specification of User Equipment (UE) procedure in idle mode, in cell reselection evaluation process, if the UE receives RRCConnectionReject message with deprioritisationReq, the UE shall consider current carrier frequency and stored frequencies due to the previously received RRCConnectionReject message with deprioritisationReq or all the frequencies of E-UTRA to be the lowest priority frequency while t325 is running irrespective of camped RAT.
Absolute priorities of different E-UTRAN frequencies or inter-RAT frequencies may be provided to the UE in the system information, in the RRCConnectionRelease message, or by inheriting from another RAT at inter-RAT cell (re) selection. In the case of system information, an E-UTRAN frequency or inter-RAT frequency may be listed without providing a priority (i.e. the field cellReselectionPriority is absent for that frequency). If priorities are provided in dedicated signalling, the UE shall ignore all the priorities provided in system information. If UE is in “camped on any cell” state, UE shall only apply the priorities provided by system information from current cell, and the UE preserves priorities provided by dedicated signalling unless specified otherwise. When the UE in “camped normally” state has only dedicated priorities other than for the current frequency, the UE shall consider the current frequency to be the lowest priority frequency (i.e. lower than the eight network configured values). While the UE is camped on a suitable CSG cell, the UE shall always consider the current frequency to be the highest priority frequency (i.e. higher than the eight network configured values), irrespective of any other priority value allocated to this frequency.
However, there is a problem associated with deprioritisationReq at public land mobile network (PLMN) change. The PLMN may have several RATs (e.g. Universal Mobile Telecommunications System (UMTS) and Long-Term Evolution (LTE)). An operator can be identified by a PLMN code, which consists of a Mobile Network Code (MNC) and Mobile Country Code (MCC) pair. In the prior art above, the UE stores the deprioritisationReq received from a first PLMN until t325 expiry, irrespective of any other cell reselection absolute priority assignments (by dedicated or common signalling), or RRC connections in E-UTRAN or other RATs. When the UE camps on a second PLMN, the UE may still de-prioritize a frequency of the first PLMN where the UE receives the deprioritisationReq in the RRCConnectionReject message if t325 does not expires. If the frequency is also used by the second PLMN and the second PLMN prefers the UE to camp on that frequency using configuration broadcast by system information, the UE cannot follow the configuration of the second PLMN until t325 expires. This makes the second PLMN cannot efficiently control the UE performing cell reselection. For example, the UE would be unable to get LTE service if the second PLMN can provide the LTE service since the frequency(s) for the LTE service may be unfortunately de-prioritized when the UE was in UMTS service providing by the first PLMN previously. Therefore, there is a need to solve the above problems.